Molecular detection and correction of 0rnithine transcarbamylase

The mitochondrial matrix enzyme ornithine trans- carbamylase (OTC; EC 2.1.3.3) catalyses the second step of the urea cycle, the conversion of ornithine and carbamyl phosphate to citrulline (Fig. 1). The OTC structural gene is located on the short arm of the X chromosome at p21.1. Deficiency of this enzyme is a common cause of a urea cycle disorder in humans and leads to the accumulation of ammonia, causing severe neurological disturbances 1. In cases of severe deficiency, this manifests itself as lethargy, vomiting and coma, starting soon after birth. Typically, affected male in- fants are well for a short interval after birth ranging from one to several days, but then they rapidly deteri- orate, becoming comatose with blood ammonia levels usually exceeding 1000 ~ti. Plasma glutamine levels are also elevated. Additional biochemical markers of the disorder help to distinguish it from other urea cycle enzyme deficiencies. For example, plasma cit- rulline levels are very low in OTC-deficient patients, often below the limit of detection by amir.o acid chro- matography. In addition, these patients excrete large amounts of orotic acid and orotidine in their urine. This orotic aciduria is due to the intramitochondrial accumulation of carbamyl phosphate, which diffuses into the cytoplasm and is converted to orotic acid by the pyrimidine synthesis pathway. Thus, the triad of hyperammonemia, low plasma citrulline, and orotic aciduria is the biochemical hallmark of this disorder. Final confirmation of the diagnosis is obtained by measuring enzyme activity in a liver specimen. Although most patients suffer from the typical neo- natal hyperammonemic crisis, males with milder enzyme defects have been described, probably repre- senting heterogeneity of the disorder at the molecular level. These patients become ill later in life and